1. Field of the Invention
This invention relates generally to gas turbine engine nozzles and, more particularly, to methods and apparatus for sealing gas turbine engine nozzles with segmented flap basesheets.
2. Description of Related Art
Military aircraft gas turbine engines often include variable geometry exhaust nozzles. The variable geometry exhaust nozzle varies throat and exit areas of the exhaust nozzle using flaps and seals. Examples of such engines are the General Electric F110 and the F414. The flaps and seals define the flowpath and the seals as their name implies seal against adjacent flaps. Because the exhaust nozzles are subjected to high temperatures and thermal gradients as a result of hot combustion gases exiting the engine, the variable geometry nozzle must maintain a coherent flowpath while shielding nozzle structural components.
Diverging flaps incorporating backbones to secure the basesheets have been used for the divergent section of convergent/divergent variable geometry nozzles. Accordingly, a center portion of each flap basesheet is unshielded and exposed to the combustion gases. During engine operation, the flap basesheet edges are exposed to less heat than the center portion of the basesheet, and as a result, a circumferential thermal gradient may be induced to the basesheet. Continued operation with the thermal gradient may induce thermal stresses into the basesheet which over time, may lead to warping, thermally induced distortion, cracking, or premature failure of the flaps.
To facilitate reducing thermal stresses induced to the flaps, at least some known engines include a “floating” basesheet design. More specifically, within such designs, a unitary basesheet is used to establish a portion of the flowpath. Because the basesheet is not rigidly joined to the backbone, the basesheet may thermally expand more than the backbone, thus facilitating reducing thermal gradients in comparison to designs having basesheets that are attached rigidly to or formed integrally with the backbone. However, because the center portion of the associated flap basesheets are still exposed to the hot combustion gases, thermal gradients between the basesheet edges and basesheet center portion may cause warping, cracking, or thermally induced distortion within the flap assembly.
One known flap incorporates a longitudinally segmented basesheet design having a central portion connected to the edge portions by channels which are crimped on the basesheet. The channels extend across and are attached to a backside of the basesheet and facilitate reducing thermally induced stresses by permitting differential thermal growth of the predominately cold portion of the seal assembly and the predominately hot portion of the segmented basesheet. However, aligning the basesheets while attaching the channel to the backside of the basesheets may be time consuming. Furthermore, because the channel is attached across the basesheet and between the edges of the basesheet, continued thermal cycling may create local stress concentrations between the channel and the basesheet.
A method for assembling a gas turbine engine variable exhaust nozzle having flaps with backbone and basesheet assemblies is disclosed in U.S. patent application Ser. No. 10/061,618, entitled “Methods and Apparatus for Sealing Gas Turbine Engine Nozzles”, filed Feb. 1, 2002, and published Aug. 7, 2003. The method includes providing a flap basesheet having a width defined between a pair of side edges extending between a leading edge and a trailing edge. At least one stiffener extends between the basesheet side edges and includes an intermediate portion that has a width that is smaller than that of the basesheet. The stiffener is bonded to or formed integrally with the basesheet. The basesheet is mounted to the gas turbine engine exhaust nozzle with a backbone assembly. Other embodiments are described in the publication.